Across a migratory divide: divergent migration directions and non-breeding grounds of Eurasian reed warblers revealed by geolocators and stable isotopes

Migratory divides represent narrow zones of overlap between parapatric populations with distinct migration directions and, consequently, expected divergent non-breeding distributions. The composition of the mixed population at a migratory divide and the corresponding non-breeding ranges remain, however, unknown for many Palaearctic-African migrants. Here, we used light-level geolocation to track migration direction and non-breeding grounds of Eurasian reed warblers (Acrocephalus scirpaceus) from three breeding populations across the species’ migratory divide. Moreover, by using feathers grown at non-breeding grounds, we quantified stable isotope composition for individuals with known southwestern (SW) and southeastern (SE) migration directions. On a larger sample per population, we then assessed the proportions of SW- and SE-migrating phenotypes in each of the three populations. All tracked reed warblers from Germany and two thirds of the birds tagged from the Czech population headed in...